1. Field of the Invention
The present invention relates to a gas supply system used in an apparatus for heat-treating semiconductor wafers under reduced pressure, and more particularly, to a gas supply system for a reduced-pressure CVD apparatus.
2. Description of the Related Art
With the improvement of the packing density and performance of modern semiconductor devices, micro-contamination has increased its influences upon the yield, quality, and reliability of the products. The micro-contamination is a trouble, such as a pattern defect caused by adhering particles or deterioration of electrical properties attributable to inclusion of heavy metal ions on the atomic or molecular level, which may cause defects in products. In order to eliminate such micro-contamination, it is necessary to prevent particles or heavy metal ions from being produced in a transportation system, treatment system, etc., included in a series of manufacturing stages for semiconductor devices, thereby totally cleaning the plant.
Since chlorine system gas, dichlorosilane gas, or some other highly corrosive gas flows as a process gas in a gas supply system of a reduced-pressure CVD apparatus, for example, a material with a high corrosion resistance is used for a gas piping. A conventional gas piping is formed of JISSUS316L, an austenitic stainless steel.
When the reduced-pressure CVD apparatus is stopped, however, nonreactive residual gas is sucked into the gas piping as the inside of a process tube is restored from the decompressed state to the atmospheric-pressure state, so that the piping is corroded by the gas. In particular, bend portions and welded joints of the piping are seriously corroded, and the resulting corrosion products form particles The welded joints of the piping produce heavy metal ions in no small quantities. If these contaminants get into the process tube and adhere to semiconductor wafers, the micro-contamination is caused.
As the memory capacity of DRAMs increases to 16M or 32M or further to 64M, the devices tend to have patterns with a submicron-order stroke width. In manufacturing these semiconductor devices with very fine patterns, micro-contamination is caused if only a small quantity of particles or heavy metal ions get into the process tube. In other words, a negligible quantity of particles or heavy metal ions for the manufacture of 4M-capacity DRAMs is a significant quantity for the manufacture of DRAMs of 16M or larger capacity. Since the conventional gas piping, formed of JISSUS316L, is not designed for the manufacture of the DRAMs of 16M or larger capacity, the quantity of particles or heavy metal ions produced at the welded joints of the pipe line cannot be ignored.
In a conventional gas supply system, as shown in FIG. 1, gas filters 7 are attached to a piping 3, and are used to remove particles in a gas supply source (not shown). A base frame 4 has a vertical panel portions 5 on each end thereof, and a piping connecting portion 3a is attached to the panel portion 5. A valve 8 and a mass-flow controller (MFC) 9 are arranged individually on pedestals 6.
The gas filters 7 are lightweight devices which each have a filter portion, formed of a stainless-steel mesh, therein. Thus, the filters 7 are removably connected to the piping 3, and are supported by means of the piping 3 only.
In a clean room which houses the gas supply system, mechanical vibrations of an air conditioner, exhaust pump, etc. are propagated to the system. Thereupon, the vibrations are propagated to the gas piping 3. If the vibration frequency agrees with the natural resonance frequency of the piping 3 or gas filters 7, a high resonance is brought about, so that the filters 7 undergo a substantial vibration. Recently, there has been an increasing demand for compact-design systems. Further, there is a proposal that a gas feeding system and an exhaust system of a heat treatment apparatus should be integrated into a unit. In this case, the piping 3 of the gas supply system inevitably has a complicated three-dimensional configuration, so that it cannot be easily fixed, and its vibration presumably becomes stronger. Accordingly, some of the particles once seized by the gas filters 7 are released by the vibration, and get into the process tube.
In general, the internal space of a semiconductor device manufacturing plant is divided in two, a clean room and a maintenance room. A semiconductor wafer transportation system and the like are installed in the clean room, and a gas feeding system and an exhaust system in the maintenance room. Since filming or etching semiconductor wafers requires use of a highly hazardous gas, combustible or toxic, a gas cylinder is located outside the plant building.
Since the gas sealed in the gas cylinder is under a very high pressure, the gas pressure is generally reduced to about 50 kg/cm.sup.2 by means of a pressure regulator which is connected directly to the cylinder. The gas pressure is also reduced by means of a pressure regulator which is attached to the piping, and the gas is fed into the process tube of the CVD apparatus via the valve the mass-flow controller, etc. In forming a polysilicon film by the reduced-pressure CVD method, for example, the pressure and flow rate of the process gas are adjusted to 2 to 3 kg/cm.sup.2 and several hundreds to several thousands of SCCM, respectively.
Since the number of processes is increased in the modern semiconductor manufacturing method, the whole system is expected to be reduced in size. Also for the gas supply system, there is a proposal to integrate the gas filters and valves into a unit, in order to ensure a compact design, easy maintenance, and general-purpose properties of the system.
Since the treatment processes are diverse, on the other hand, the gas piping 3 of the gas supply system has many branches, and many piping accessories are arranged for each branch. If the piping accessory unit is constructed in the aforesaid manner, therefore, the piping in the unit is long, and the piping accessories are increased in number.
As a safety measure for handling the hazardous gas, in the conventional semiconductor device manufacturing plant, however, large-sized, large-capacity gas filters and valves, which can stand high pressure, are used for the conventional gas supply system. For example, pressure valves which can stand a pressure much higher than 100 kg/cm.sup.2 are used for the purpose. Thus, the piping unit is increased in size and weight.